1. Field of the Invention
The present invention relates to a liquid crystal display device and, in particular, a technology effectively applied to in-plane switching (IPS) liquid crystal display devices.
2. Description of the Related Art
In liquid crystal display devices that are mounted to portable electronic devices such as cell phones and digital cameras, the number of pixels is increasing progressively and the areal dimensions per pixel tend to decrease accordingly. On the other hand, the demand for high image quality is still strong, and those liquid crystal display devices are required to balance high image quality and high pixel count. Major factors that give a liquid crystal display device high image quality are brightness, contrast ratio, and viewing angle.
An IPS liquid crystal display device has, in each pixel, an electrode that is characterized by being shaped in a stripe pattern in plan view, and drives a liquid crystal layer by applying an electric field whose major components are parallel to the substrate plane (so-called lateral electric field) to the liquid crystal layer. When no electric field is applied, the liquid crystal layer of the IPS liquid crystal display device has a homogenous alignment in which the long axis direction (molecular axis direction) of liquid crystal molecules is substantially parallel to the substrate plane. Applying a voltage causes a change in the liquid crystal layer that rotates the alignment substantially within the substrate plane. Excellent display characteristics in terms of viewing angle are therefore obtained. IPS liquid crystal display devices are particularly superior in tone reproduction in the viewing angle direction.
When a lateral electric field is applied to the liquid crystal layer having a homogeneous alignment, there are two possible rotation directions, clockwise and counter-clockwise, for the alignment of the liquid crystal layer. The liquid crystal layer can be prompted to rotate the alignment in one of the two possible directions by setting the direction of the electrode stripes (i.e., electric field direction) and the initial alignment direction of the liquid crystal layer suitably. In other words, when the alignment direction of the liquid crystal layer rotates with the initial alignment direction as a starting point, a rotation direction that requires a smaller rotation angle to be parallel to the electric field direction is chosen. A uniform alignment state is therefore obtained, at least at the center of each pixel, even when a voltage is applied.
Among IPS liquid crystal display devices, IPS provectus (IPS-Pro) liquid crystal display devices have excellent transmittance and are therefore capable of brighter display. IPS-Pro liquid crystal display devices accomplish high transmittance by disposing a pixel electrode and a common electrode in different layers, and shaping one of the two electrodes that is disposed close to a liquid crystal layer in a stripe pattern in plan view, while giving the electrode that is disposed far from the liquid crystal layer a solid flat shape in plan view. Conventional IPS-Pro liquid crystal display devices usually choose the pixel electrode as the electrode that is disposed close to the liquid crystal layer. However, in many of IPS-Pro liquid crystal display devices that have been proposed in recent years, the common electrode is chosen as the electrode that is disposed close to the liquid crystal layer, the stripe structure stretches over a plurality of pixels, and the direction of the stripes is set in the direction of the shorter sides of the pixels (see JP 2007-256905 A, for example).